As a tire on a road wheel rotates, only a portion of the tire makes contact with a travel surface (e.g., a road). The portion of the tire that makes contact with the travel surface is often referred to as a tire contact patch. When a driver of a vehicle desires to turn the vehicle, the driver typically turns a handwheel to change the lateral travel direction of the vehicle, resulting in vehicle yaw. The vehicle's tires supply the necessary force to turn the vehicle, referred to as tire lateral force. Due to the elastic nature of a tire sidewall and friction between the tire contact patch and the travel surface, as the angle of the handwheel changes during a turn, there may be an angular difference between the direction of travel of the turning tire's contact patch and the direction of travel of the road wheel of winch the turning tire is a part, known as a tire slip angle.
In the past, electronically controlled steeling systems used control algorithms that did not fully comprehend the relationship between tire lateral force and tire slip angle (tire force-slip relationship). The tire force-slip relationship is typically a nonlinear function, where tire lateral force saturates at large positive and large negative tire slip angles. The tire force-slip relationship varies based on factors such as the coefficient of function between the tire contact patch and the travel surface, as well as other variables including particular tire and vehicle characteristics. Early steering control systems made no attempt to include the tire force-slip relationship in compensation algorithms. Such systems allowed for saturation of tire lateral force, resulting in reduced vehicle steering performance.
Other steering control systems have been contemplated that attempt to control tire slip angle based upon an assumed relationship where a peak or knee value appears near the saturation region of a tire force-slip curve. Such systems attempt to maintain the tire slip angle in a region between the peak and a maximum tire slip angle. These systems also attempt to change the peak and maximum tire slip angles based upon an estimated coefficient of friction of the travel surface. However, such systems fail to account for common conditions where there is no knee or peak in a tire force-slip curve, therefore limiting the usefulness of such systems to a narrow range of conditions.
Furthermore, such steering control systems also fail to account for the contributions from steering control system augmentation offsets, such as offsets produced by a stability control algorithm. When steering control system augmentation offsets are included in a limiting system, the order of limiting calculations is critically important. If a steering actuator is adjusted based on tire force-slip calculations alone, an over or under correction may result, as steering control system augmentation offsets are also added or subtracted from the steering angle. In failing to account for the effects of additional contributions from steering control system augmentation offsets, such steering control systems may allow tires to reach excessively large slip angles where lateral force is saturated.
Such steering control systems also require multiple sensors, which can drive up overall system cost substantially. Sensors required by such steering control systems may include one or more: yaw rate sensors, speed sensors, lateral acceleration sensors, roll rate sensors, steering angle sensors, longitudinal acceleration sensors, pitch rate sensors, and steering angle position sensors. The high cost that results from such systems may make the initial investment prohibitively expensive for manufacturers and consumers alike. Additionally, the large number of sensors, associated wiring, harnesses, and support assemblies may increase overall vehicle weight, yielding a reduction in vehicle fuel efficiency. Therefore, it would be beneficially to lessen the total number of required sensors for a steering control system, while accounting for the tire force-slip relationship.
The shortcomings of the aforementioned steering control systems may put drivers and passengers of vehicles at risk. Large tire slip angles may develop during emergency maneuvers on any travel surface and may also occur during moderate maneuvers on slippery travel surfaces. The resulting saturation of lateral forces from the large tire slip angles leads to a loss of vehicle response and excessive oversteer (i.e., spin-out) or understeer (i.e., plowing) of the vehicle. Since the previous electronically controlled steering systems did not fully recognize the nonlinear force saturation characteristic of tires, the additional effects of steering control system augmentation offsets, and the potential variety in tire force-slip curves, they were unable to prevent excessive understeer or oversteer of the vehicle in situations when precise control was needed most.
Accordingly, there is a need for a method that overcomes these drawbacks and properly accounts for the nonlinear relationship between tire lateral force and tire slip angle to mitigate excessive understeer or oversteer of a vehicle.